1. Field of the Invention
The present invention relates to a printed dual-band Yagi-Uda antenna and a circular polarization antenna, and more particularly, to a printed dual-band Yagi-Uda antenna with a high directivity radiation pattern and a circular polarization antenna for a multi-input multi-output (MIMO) wireless communication system.
2. Description of the Prior Art
In modern life, various wireless communication networks have become essential for people to exchange voices, text messages, data, video files, etc. Since antennas are required for accessing these wireless communication networks with information carried by electromagnetic waves, development and research of the antennas have become one key issue for modern information technology manufacturers. In order to realize compact portable wireless communication devices, such as cell phones, personal digital assistants (PDAs), wireless USB dongles, the size of antennas should be implemented as small as possible, such that the antennas can be integrated into the portable communication devices.
Due to merits such as light weight, small size, and high compatibility with various circuits, a printed antenna is widely used for all kinds of wireless communication products. Generally speaking, in order to reduce blind angles of signal emission or reception, the printed antenna of the wireless communication product is mostly implemented by an omni-directional antenna, such as a dipole antenna. In a horizontal plane, signals of the omni-directional antenna radiate in 360 degree and have little variation in short distance, and thus the omni-directional antenna is suitable for practical applications. However, with introduction of an antenna array or a smart antenna technology, a single antenna is often required to have a high gain and high directivity radiation pattern. In such a condition, a printed Yagi-Uda antenna is proposed, which utilizes high directivity of the Yagi-Uda antenna to enhance antenna gain on an operating frequency band, such that communication quality can be improved.
Please refer to FIG. 1, which is a schematic diagram of a conventional Yagi-Uda antenna 10. The Yagi-Uda antenna 10 has a most basic structure of a Yagi-Uda antenna, and consists of three components: a driver 11, a reflector 12 and a director 13. The driver 11 is generally realized by a dipole antenna, and is utilized for producing resonance according to a fed time-varying current to generate a radiation field. The reflector 12 and the director 13 are formed by sheet metals or plate metals, and are utilized for exciting an in-phase and an anti-phase radiation electric field through electromagnetic coupling, respectively. As a result, the reflector 12 and the director 13 can reflect or direct the radiation patterns generated by the dipole antenna toward a specific direction, so as to enhance antenna gain. Of course, the number of parasitic elements such as the reflector and the director can be adjusted according to practical antenna gain requirements, which is known by those skilled in the art and therefore not detailed here.
In addition, a circular polarization antenna can be utilized for avoiding polarization dependent loss resulted from polarization mismatch between a transmission antenna and a reception antenna. Therefore, a receiver can be placed with more flexibility in practical applications. However, a normal circular polarization antenna is usually implemented in a single-band system structure, such as a satellite communication system, and does not have a high directive radiation pattern, so that requirement of current wireless communication product is hard to meet.
Besides, with advancement of wireless communication technologies, the number of antennas equipped for the electronic product is increased. For example, a multi-input multi-output (MIMO) communication technology is supported by IEEE 802.11n. That is, a related electronic product can simultaneously transmit and receive radio signals by use of multiple antennas, such that data throughput and transmission distance can be significantly increased without extra bandwidth or power expenditure. Thus, spectral efficiency and transmission rates of the wireless communication system can be enhanced, so as to improve communication quality.
However, the conventional printed Yagi-Uda antenna is a single band antenna, and can not meet multi-band requirements in current wireless communication products. In addition, each antenna of the conventional MIMO system has a fixed polarization direction and can not be adjusted according to system requirements, causing transmission efficiency is likely affected due to polarization mismatch. Thus, there is a need to improve.